In the semiconductor integrated circuit ("IC") industry, an IC product concept is first implemented by designing circuit schematics. To realize the designed circuit, a mask layout comprising a series of layers of patterns is generated by layout engineers. Each layer of patterns is then made into a photolithographic mask, which is later used to fabricate a semiconductor chip embodying the designed circuit.
An integrated circuit is made using a sequence of processes performed on a semiconductor substrate. The processes performed on the substrate include some or all of the following: chemical and physical film depositions, etching, ion implantation, diffusion, annealing or thermal oxidation. Many of these processes require that a pattern of photoresist first be formed on the substrate. The process for patterning the photoresist is called a photolithographic process--it involves first depositing a uniform layer of photoresist on the substrate, next exposing the photoresist layer to optical illumination through a mask, and then developing the exposed photoresist layer. The photoresist layer that results from this process is patterned to form an image that corresponds to the patterns on the mask. Depending on the type of photoresist used, the image is either a positive one or a negative one.
A conventional mask consists of a thin layer of chromium (about 50 nanometers) deposited and then patterned on a glass or quartz substrate. In the photolithography process, the mask pattern size is reduced by a factor of 5 after it is transferred onto the substrate by a lithographic exposure system. The pattern is binary; that is, it is either opaque or transparent. In a conventional optical lithography system, the image resolution and the depth of focus are determined by the wavelength of the illumination light and the numeric aperture of the optics and not by the mask itself.
In contrast with the binary mask, a phase-shift mask contains phase shifters to enhance the resolution of the mask image, which results in an increased useful lifetime for conventional lithography systems. One kind of phase-shifter is comprised of tiny apertures formed along the peripheries of a chrome mask pattern and a layer of transparent material formed on the apertures. The thickness of the transparent material is such that light passing through the aperture is 180 degrees out of phase with the light passing by edges of the chrome mask pattern. As a result of the interference between the light passing through the apertures and the light passing by the edges, the contrast of the projected mask image is enhanced.
In the last 20 years, lithography innovation has been aimed at improving resolution and alignment accuracy, increasing throughput and reducing defect density. As IC technology advances, the number of layers in an IC has increased and, as a result, the number of photolithographic processes required to fabricate an IC has also increased. Owing to the increasing number of masks needed to make an IC and stringent defect requirements, each set of masks is becoming very expensive. For low volume ICs or prototypes, the mask could be the dominant cost. This becomes an important consideration for prototyping new ICs. Whenever a design modification is required, a new mask must be generated, resulting in increased cost and development cycle time. It is also uneconomical to merge several different mask sets onto a single wafer lot. Thus, the cost of throughput reduction due to loading, alignment, and unloading of masks far out-strips reduction in processing cost.
Therefore, there remains a need for lithography innovation that minimizes prototyping cost and development cycle time. An E-beam (electron beam) direct write system can potentially achieve much of the desired characteristics. In an E-beam system, because the layout is stored in a computer and the photoresist is directly written by an exposure e-beam, no mask is needed. This eliminates the mask cost and the time required to generate the mask. Design modification can be more quickly implemented. In addition, different prototyping products can be merged onto the same wafer lots without reduced throughput. The processing cost, spread among the prototypes on the same wafer lot, can be significantly lowered.
Notwithstanding the aforementioned advantages, the throughput of E-beam lithography is too low to be competitive with conventional optical lithography. It is therefore rarely used except in making masks.
In contrast to the E-beam exposure system, the conventional chrome mask is rigid and inflexible. Except in cases where defects or errors are very limited, repair of the mask is complicated, difficult, and time consuming. Once it is made, a mask's pattern is considered fixed; it is not possible to alter the pattern at will. Previously known optical lithography systems lack the flexibility afforded by E-beam exposure systems.
It is therefore an object of the present invention to provide a mask having a mask pattern that can be altered at will;
it is another object of the present invention to provide a mask that can be used to form all of the mask patterns that are needed in making a semiconductor or IC; PA1 it is yet another object of the present invention to reduce the cost and the time for forming a mask pattern; PA1 it is a further object of the present invention to provide a mask wherein layout modification and error correction can be implemented quickly and inexpensively; PA1 it is a still further object of the present invention to provide a phase-shift mask having a mask pattern that can be altered at will; and PA1 it is a further object of the present invention to provide a mask wherein the mask pattern can be modified using exposure modulation to compensate for imperfections of the projected mask image.